Basic technical contents of a room-temperature bonding method based on a surface activation method are disclosed in Japanese Laid-Open Patent Application JP-A-Showa 62-22712 (Japanese Patent JP1422397B) and Japanese Patent JP2791429B. However, since there are some materials difficult to bond together at a room temperature even by using this method, a bonding method in such a case is disclosed in following documents.
For example, after ion beams or atom beams using Ar or the like are irradiated onto surfaces to be bonded not only for removing contamination layers on the surfaces to be bonded but also for activating the surfaces to be bonded, an active ultra fine particle film is further formed on each of the surfaces to be bonded. At this time, as a method of forming an active layer, a target material is sputtered to thereby form the active ultrafine particle film on the surface to be bonded, opposed to the target. However, it is described that because of a low density of the ultrafine particle film, it is preferable to heat the surfaces so as to improve a degree of sintering during bonding (Japanese Laid-Open Patent Application JP-A-Heisei 6-99317).
Furthermore, various types of metal films each having good adhesiveness can be formed on diverse materials by using active Ti, Cr or the like as an intermediate layer. By using this intermediate layer to form a metal film on a surface of a material such as SiO2 for which sufficient adhesion strength cannot be obtained by direct bonding based on the surface activation method, it is expected to be able to improve a performance of bonding of the material. Si, SiO2 and LiNbO3 are disclosed as examples of the material, and Pt as well as Ti and Cr is disclosed as an example of a material of the intermediate layer (Report of Mechanical Engineering Laboratory, No. 189, Chapter 8, December, 2000).
It is disclosed that irradiating inert gas ion beams or inert gas neutral atomic beams and metal ion beams or metal neutral atomic beams onto surfaces to be bonded of two substrates in vacuum to thereby form a metal thin film having a film thickness of 1 nm to 100 nm on each of the surfaces to be bonded of the substrates so as to bond, at a low temperature, the substrates having surfaces composed of ionic crystals and difficult to bond together by the conventional surface activation method. FIG. 6 shows an example of the irradiation. A SiO2 substrate 107 is used as the substrate to be bonded, iron atoms 106 are used for the metal neutral atomic beam and argon atoms 105 are used for the inert gas neutral atomic beam. Each of the beams is irradiated from a beam irradiation unit 102 that includes therein an electrode 101, a beam source 103 and a grid 104. Specifically, the iron atoms 106 are generated as follows. The grid 104 composed of iron is disposed in an opening portion of the beam source 103, apart of argon atomic beams generated by applying a voltage to the electrode 101 from the beam source 103 strikes against the grid 104, and a part of the struck grid 104 is sputtered. Quartz, silicon oxide, silicon nitride, sapphire and LiNbO3 are disclosed as examples of the ionic crystal, and iron, titanium and gold are disclosed as examples of a type of the metal. If the grid is replaced by another grid, the type of the metal can be changed. This method is characterized in that surface activation of the surface to be bonded of the substrate and deposition of a metal thin film are simultaneously performed. Therefore, by adjusting energy of either the ion beams or the neutral atomic beams, it is controlled to preferentially perform an action of activation or an action of metal thin film deposition. However, to improve a bonding strength, it is necessary to heat each substrate to 150° C. to 500° C. and to press the substrates by a roller during bonding. It is also necessary to produce another grid so as to change the type of the metal thin film. It is, therefore, not easy to change the type of the metal thin film (Japanese Laid-Open Patent Application JP2004-337927A).
A method of accelerating bonding is disclosed in which a metal thin film layer or an amorphous layer is formed on a bonding interface. This method is to bond a polycrystalline substance to a single-crystalline substance where it is difficult for both of the crystalline substances to realize a smooth surface roughness. The metal thin film layer is formed on a surface to be bonded of the polycrystalline substance, and the amorphous layer containing an element constituting the single-crystalline substance or an element of a metal constituting the metal thin film layer is formed on a surface to be bonded of the single-crystalline substance. Thereafter, the surfaces are activated and bonded to each other. Sintered ceramic is disclosed as the polycrystalline substance, and Si, SiGe and GaAs are disclosed as the single-crystalline substance. Pt (platinum) and Au (gold) are disclosed as the metal. As a method of forming the metal thin film, a sputtering film formation method is disclosed which uses an ion beam obtained by irradiating plasma ions onto a target and using recoil particles as film formation particles. This method is a method of bonding the substances by the amorphous layer produced by a reaction between the metal and the semiconductor, and application of the method is limited to semiconductor substrate materials (Japanese Laid-Open Patent Application JP2005-104810A).